Process for producing foamed body of thermoplastic resin, mold for forming same and foamed body of thermoplastic resin

ABSTRACT

The invention provides a foamed body of thermoplastic resin having a high expansion ratio, desired shape and cells of desired diameter, especially fine cells. The foamed body of thermoplastic resin is produced by filling the thermoplastic resin containing a foaming agent into a cavity  31  of a mold  2   a  and thereafter enlarging the cavity 31 to expand the resin. In this process, the cavity  31  is completely filled with the resin as melted and containing the foaming agent, then held in its shape as filled with the resin and enlarged to an increased final cavity width upon the average temperature filled in the cavity  31  reaching the range from the melting point of the resin to the melting point plus 30° C.

TECHNICAL FIELD

The present invention relates to a process for producing foamed bodiesof thermoplastic resin by filling the cavity of a mold with thethermoplastic resin as melted and containing a foaming agent andthereafter enlarging the cavity, and to molds for use in the process forproducing the resin foamed body. The invention relates also to foamedbodies of thermoplastic resin which have high compressive rigidity andhigh flexural rigidity in the direction of thickness.

BACKGROUND ART

As already disclosed, for example, in the publication of JP-B No.51-8424(1976), a process is known for producing a foamed body ofthermoplastic resin by filling the cavity of a mold with thethermoplastic resin as melted and containing a foaming agent andthereafter enlarging the cavity. As shown in FIG. 12 by a line 91, thecavity is enlarged in this process at a rate of V2 for a period of timeT2 after the completion of filling.

Further known is a process wherein the cavity is enlarged at differentrates to obtain a foamed body having a desired feel to the touch bycontrolling the thickness of a nonexpanded skin layer as disclosed, forexample, in the publication of JP-A No. 7-88878(1995). With thisprocess, the cavity is enlarged at a rate of V1 until time T1 after thecompletion of filling and then at a lower rate V3 from time T1 to timeT2.

The publications of JP-B No. 51-8424(1976) and JP-A No. 6-198668(1994)disclose a process wherein the cavity of a mold used is enlarged forforming a foamed body of thermoplastic resin. This process prepares anexpanded molding by filling the cavity with the thermoplastic resin asmelted and containing a foaming agent and thereafter forcing the cavityto enlarge rapidly by a predetermined amount.

The foamed body is produced by this process by filling the thermoplasticresin containing a foaming agent into the cavity as held diminished fromthe final shape thereof and subsequently enlarging the cavity to thesize of the final product.

However, the conventional processes for producing foamed bodies ofthermoplastic resin have the problem that it is difficult to obtainfoamed bodies of great expansion ratio, for example, those havingheat-insulating properties and an expansion ratio of at least 5 times.Another problem encountered is that the conventional process affordsonly those having a definite cell diameter as large as at least severalmillimeters, producing only hollow moldings in the case of unsuitedconditions.

Additionally the conventional processes have the problem that the resinfails to fully expand in the cavity corners in conformity with theenlargement of the cavity. Stated more specifically, when a resin 104containing a foaming agent is filled into a cavity 103 defined by afixed mold 101 and a movable mold 102 as shown in FIG. 13(a) and whenthe cavity 103 is thereafter enlarged by moving the movable mold 102toward the direction of arrow 107 as shown in FIG. 13(b), it is likelythat the thermoplastic resin 104 will fail to expand in the corners ofthe cavity 103 in conformity with the enlargement of the cavity 103,consequently affording only a foamed body 106 which is recessed atcorners 105.

An object of the present invention, which is accomplished in view of theforegoing problems of the prior art, is to overcome these problems andto provide a process for producing a foamed body of thermoplastic resinof desired shape having a high expansion ratio and containing fine cellsof desired diameter, and a mold for use in the process for formingfoamed bodies of thermoplastic resin.

On the other hand, the publication of JP-A No. 10-230528(1998) disclosesa process for producing a foamed body having a surface layer ofnonexpanded portion integral therewith and satisfactory in surfaceappearance and having fine closed cells with a uniform average celldensity, using carbon dioxide or nitrogen in a supercritical state as afoaming agent.

The publication of JP-A No. 8-108440(1996) discloses an expanded boardof polyolefin resin having a cell structure of closed cells (a) at least85% of which are 2.5 to 10.0 in the ratio of the size thereof in adirection perpendicular to the plane of the board to the size thereof ina direction parallel to the board, and (b) at least 70% of which are upto 500 μm in size in a direction parallel to the plane of the board, theexpanded board being at least 2 in the ratio of the compressive strengththereof in a direction perpendicular to the plane of the board to thecompressive strength thereof in a direction parallel to the plane of theboard, and 5 to 20 times in expansion ratio. This expanded board ofpolyolefin resin is produced from a mixture of the polyolefin resin, achemical foaming agent and a crosslinking agent by applying pressure tothe mixture using molds having opposed parallel inner surfaces, heatingthe mixture at a temperature not lower than the decompositiontemperature of the foaming agent, and subsequently increasing thedistance between the molds to expand the polyolefin resin only in adirection perpendicular to the inner surfaces through which the pressureis applied. This process involves ingenuity to produce a flatted cellstructure to achieve both a low density and high compressive rigidity.

Since the production process of JP-A No. 10-230528(1998) is an injectionmolding process, the foamed body obtained is nevertheless low incompressive strength and in properties to absorb great impact because ofits structure of fine and uniform closed cells although having shapeconformability. The product therefore needs to be given a higher densityso as to be improved in physical properties, whereas this entails theproblem of making the molding heavier and impaired in heat-insulatingproperties.

The molding of JP-A No. 8-108440(1996) consists solely of an expandedlayer providing a single-layer structure and is therefore low inflexural strength and small in deformation limit due to the presence ofclosed cells. The process as described above also has the problem ofaffording moldings only in the form of a board. The use of the chemicalfoaming agent results in an increased cost and permits the unreactedcomponents and decomposed products of the agent to remain in the foamedbody, consequently entailing discoloration of the foamed body, releaseof odor therefrom and problems of food hygiene.

In view of the above situations, another object of the present inventionis to provide a foamed body of thermoplastic resin which has highcompressive rigidity and high flexural rigidity in the direction ofthickness thereof, lightweight, outstanding in heat-insulatingproperties and impact absorbing properties and free from the residues ofchemical foaming agent.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished to fulfill the aboveobjects. The invention provides as a first feature thereof a process forproducing a foamed body of a thermoplastic resin by filling thethermoplastic resin as melted and containing a foaming agent into acavity of a mold and thereafter enlarging the cavity to expand theresin, the process being characterized by holding the cavity in theshape thereof during filling after the molten resin containing thefoaming agent is completely filled into the cavity and enlarging thecavity to an increased final cavity width upon the average temperatureof the molten resin within the cavity reaching the range of the meltingpoint of the resin to [the melting point+30° C.].

The present invention provides as a second feature thereof a process forproducing a foamed body of a thermoplastic resin by filling thethermoplastic resin as melted and containing a foaming agent into acavity of a mold and thereafter enlarging the cavity to expand theresin, the process being characterized in that the process includes theprimary cavity enlarging step of enlarging the cavity to a predeterminedvalue less than an increased final cavity width upon the averagetemperature of the molten resin within the cavity reaching the range ofthe melting point of the resin to [the melting point+60° C.] afterholding the cavity in the shape thereof as filled with the molten resincontaining the foaming agent subsequently to complete filling of thecavity, and the secondary cavity enlarging step of subsequentlyinterrupting the enlargement of the cavity and enlarging the cavity tothe increased final cavity width upon the temperature of the resin inits central portion with respect to the thickness thereof reaching atemperature from the melting point to [the melting point+50° C.].

The present invention provides as a third feature thereof a process forproducing a foamed body of a thermoplastic resin, the process being onemode of practicing the process of the first feature and characterized inthat the process includes the steps of holding the cavity in the shapethereof as filled with the resin for a predetermined period of timeafter the molten resin containing the foaming agent is completely filledinto the cavity, and enlarging the cavity to an increased final cavitywidth after the predetermined period of time.

The invention provides a fourth feature thereof a process for producinga foamed body of a thermoplastic resin, the process being one mode ofpracticing the process of the second feature and characterized in thatthe process includes the primary cavity enlarging step of enlarging thecavity to a predetermined value less than an increased final cavitywidth a predetermined period of time after the molten resin containingthe foaming agent is completely filled into the cavity, and thesecondary cavity enlarging step of enlarging the cavity to the increasedfinal cavity width after interrupting the enlargement of the cavity fora specified period of time.

The invention provides as a fifth feature thereof a process forproducing a foamed body of a thermoplastic resin which is characterizedby enlarging the cavity to the increased final cavity width at a rate of2 to 5 mm/sec in a process according to the first or third feature ofthe invention, and enlarging the cavity at a rate of 2 to 5 mm/sec inthe primary cavity enlarging step in a process according to the secondor fourth feature of the invention.

The invention provides as a sixth feature thereof a process forproducing a foamed body of a thermoplastic resin according to thesecond, fourth or fifth feature of the invention, the process beingcharacterized by enlarging the cavity at a rate of 5 to 10 mm/sec in thesecondary cavity enlarging step.

The invention provides as a seventh feature thereof a process forproducing a foamed body of a thermoplastic resin according to any one ofthe first to the sixth features of the invention, the process beingcharacterized in that the foaming agent is an inert gas, namely a gaswhich is unreactive to the resin.

The term the “direction of forward movement” as used herein refers tothe direction in which the movable piece moves toward a gate portion ofthe cavity so as to diminish the cavity and therefore to the directionindicated at 51 in FIG. 4(b). The term the “direction of rearwardmovement” refers to the direction in which the movable piece moves awayfrom the gate portion of the cavity so as to enlarge the cavity, andaccordingly to the direction indicated at 52 in FIG. 4(b).

The invention provides as an eighth feature thereof a mold for forming afoamed body of a thermoplastic resin by filling the thermoplastic resinas melted and containing a foaming agent into a cavity defined by astationary mold member and a movable mold member of a mold andthereafter enlarging the cavity to expand the resin and produce thefoamed body thereof, the mold being characterized in that the movablemold member is provided with a movable piece movable forward or rearwardalong the direction of thickness of the cavity 31, the movable piecebeing so shaped as to define a peripheral cavity space 32 by an outerperipheral surface of the movable piece and a peripheral cavity wall 33and a main cavity space 34 by a front end face of the movable piece anda cavity wall 35 opposed thereto when the movable piece is movedforward.

The peripheral cavity space 32 can be in the form of an integral tubeand can be rectangular parallelepipededal or otherwise polygonal orcircular in cross section.

With reference to FIG. 4(a), a movable piece 221 a provided on a movablemold member 22 a forms a tubular cavity 31 having a bottom and, forexample, a recessed section along the direction of forward or rearwardmovement when moved forward to the greatest extent. The bottomed tubularcavity 31 comprises a peripheral cavity space 32 defined by the outerperipheral surface of the movable piece 221 a and a cavity peripheralsurface 33, and a main cavity space 34 defined by the front end face ofthe movable piece 221 a and a cavity wall 35 opposed thereto.

The invention provides as a ninth feature thereof a mold for forming afoamed body of a thermoplastic resin according to the eighth feature ofthe invention, the mold being characterized in that the length t5 of theperipheral cavity space 32 in the direction of forward or rearwardmovement is greater than the width t3 of the main cavity space 34 in thedirection of forward or rearward movement.

With reference to FIG. 4(a), it is desired that the length t5, along thedirection of forward or rearward movement, of the peripheral cavityspace 32 of the tubular cavity 31 having a bottom and, for example, arecessed section be greater than the width t3 of the main cavity space34.

The invention provides as a tenth feature thereof a mold for forming afoamed body of a thermoplastic resin according to the eighth or ninthfeature of the invention, the mold being characterized in that thelength t5 of the peripheral cavity space 32 in the direction of forwardor rearward movement is 50 to 70% of an increased final cavity width t6.

With reference to FIG. 4(a), the length t5, along the direction offorward or rearward movement, of the peripheral cavity space 32 of thetubular cavity 31 having a bottom and, for example, a recessed sectionis preferably 50 to 70%, more preferably 60 to 65%, of the increasedfinal cavity width t6.

If the length t5 is smaller than 50% of the increased final cavity widtht6, the foamed body of thermoplastic resin obtained has a peripheralportion failing to conform to the shape of the mold. It is then likelythat the product of the desired shape will not be available. If thelength t5 is greater than 70% of the increased final cavity width t6,the foamed body of thermoplastic resin obtained is likely to havewrinkles and an impaired appearance.

The invention provides as an eleventh feature thereof a mold for forminga foamed body of a thermoplastic resin according to the eighth, ninth ortenth feature of the invention, the mold being characterized in that thewidth t4 of the peripheral cavity space 32 is two to four times thewidth t3 of the main cavity space 34.

With reference to FIG. 4(a), the width t4 of the peripheral cavity space32 is 2 to 4 times, preferably 2.7 to 3.5 times, the width t3 of themain cavity space 34.

If the width t4 of the peripheral cavity space 32 is smaller than twicethe width t3 of the main cavity space 34, the peripheral portion of thefoamed thermoplastic resin body to be obtained fails to conform to theshape of the mold, and it is likely that a foamed body of the desiredshape will not be available. If the width t4 of the peripheral cavityspace 32 is greater than four times the width t3 of the,main cavityspace 34, the foamed body of thermoplastic resin will deform when cooledinsufficiently, failing to have the desired shape. An attempt to coolthe body sufficiently is likely to entail greatly lowered productivity.It is also likely that the thermoplastic resin within the peripheralcavity space 32 will not be expanded to a full extent.

The thermoplastic resin to be used in the present invention is notlimited specifically. Examples of useful resins are polyethylene,polypropylene, ethylene-propylene copolymer, ethylene-propylene-dienecopolymer, ethylene-vinyl acetate copolymer, polybutene, chlorinatedpolyethylene and like olefin resins, polystyrene,styrene-butadiene-styrene copolymer, styrene-isoprene-styrene resin,polymethyl acrylate, ethylene-ethyl acrylate copolymer and like acrylicresins, polyvinyl chloride and like chlorine resins, polyethylenefluoride and like fluorocarbon resins, 6-nylon, 66-nylon, 12-nylon andlike polyamide resins, polyethylene terephthalate, polybutyleneterephthalate and like polyester resins, ABS resin, polycarbonate,polyacetal, polyphenylene sulfide, polyether ether ketone, polyetherimide, silicone resin, thermoplastic urethane, various elastomers, etc.

Especially preferable among these examples are resins having melttension or stretch viscosity suitable for expansion. Such resinsfavorably usable are, for example, polyethylene, polypropylene,polystyrene, ABS resin, polyvinyl chloride, etc. Examples of usefulpolyethylene and polypropylene are finely cross linked resins which areadjusted in stretch viscosity characteristics.

These resins can be used singly or at least two of them can be used inthe form of an alloy, blend or composite.

The term the “melting point” is defined as the temperature at which aresin changes from a fluid state to a nonfluid state. In the case ofcrystalline resins such as polyethylene, polypropylene, polyacetal,polybutylene terephthalate, the melting temperature Tpm measuredaccording to JIS K7121 is used as the melting point. In the case ofnoncrystalline resins such as polystyrene, polyvinyl acetate, polymethylmethacrylate, polycarbonate and ABS resin, the glass transitiontemperature Tmg measured according to JIS K7121 is taken as the meltingpoint. Although vinyl chloride is a crystalline resin, the thermaldecomposition temperature thereof is approximate to the meltingtemperature Tpm thereof, and the resin is molded at a temperature of upto the melting temperature Tpm. In the case of the crystalline resinswhich are molded at a temperature of not higher than the meltingtemperature Tpm, [the melting temperature Tpm−30° C.] is taken as themelting point. Similarly, with the noncrystalline resins which aremolded at a temperature of up to glass transition temperature Tmg, [theglass transition temperature Tmg−30° C.] is taken as the melting point.

The term the “average temperature of the resin” as used herein isdefined as the average value of temperature of the resin layer filled inthe mold cavity in the direction of thickness of the layer. Although thetemperature in the direction of thickness of the resin layer is notlimited particularly, the temperature is preferably the value at themidportion of the mold gate portion and the forward end of the resinlayer, i.e., at the midportion of the thickness of the layer.

The temperature of the resin as melted and filling the mold isdetermined, for example, by measuring the temperature of the filledresin layer with an infrared temperature sensor installed in the mold,or by calculating the temperature of the filled resin layer by CAEinjection flow analysis.

The expressions “holding the cavity as filled” and “interruptingenlargement of the cavity” as used herein mean the state of the movablepiece substantially at a halt, and refer to the state in which the rateof enlargement by the movable piece is up to 0.1 mm/sec. However, therate of enlargement by the movable piece is preferably 0 mm/sec.

The foaming agent for use in the present invention is not limitedparticularly, but usable are organic or inorganic thermally decomposablechemical foaming agents or physical foaming agents. Examples of usefulchemical foaming agents are azo compounds, hydrazide compounds, nitrosocompounds, semicarbazide compounds, hydrazo compounds, tetrazolecompounds, ester compounds, bicarbonates, carbonates, nitrites, etc.More specific examples are azodicarbonamide (ADCA), isobutyronitrile(AZDN), benzenesulfonylhydrazo (OBSH), dinitropentamethylenetetramine(DPT), azobisisobutyronitrile (AIBN), p-toluenesulfonhydrazide (TSH),barium azodicarboxylate (Ba-AC), etc. Examples of physical foamingagents are carbon dioxide gas, argon, neon, helium, oxygen and likenonreactive gases. These agents can be used singly, or at least two ofthem are usable in combination.

The method of enlarging the mold cavity is not limited particularly. Forexample, the movable piece of the mold is moved toward a direction by ahydraulic mechanism for an injection molding machine as shown in FIGS. 2and 3 or an external hydraulic device and a hydraulic piston to enlargethe cavity, or the cavity is enlarged utilizing a mold opening mechanismfor an injection molding machine as shown in FIGS. 5 and 6.

The “cavity width resulting from the primary cavity enlarging step”according to the second, fourth or fifth feature of the invention is notlimited particularly if the width is not greater than the increasedfinal cavity width t6. However, the width is preferably up to 3.0 timesthe width t3 of the main cavity space 34 in the direction of forward orrearward movement, more preferably up to 6.5 mm. If the width increasedby the primary enlargement is excessively great, the cell film is likelyto break, joining large numbers of small cells into large cells.

The “rate of primary cavity enlargement step” according to the second,fourth or fifth feature of the invention and the “rate of enlargement tothe final cavity width” according to the first, third or fifth featureof the invention are preferably 0.5 to 15 mm/sec, more preferably 2 to 5mm/sec. If the cavity enlargement rate is too low, only a foamed body oflarge cells is obtained. If the rate is excessively high, the resinsurface separates from the mold once and is then transferred onto themold surface again during the subsequent step of interrupting the cavitymovement. This is likely to give rise to the problem of impairing theappearance of the surface of the molding.

According to the third feature of the invention, the period of timeduring which the cavity is held as filled with the resin immediatelyafter the completion of filling until the start of enlargement of thecavity is dependent on the temperature of the resin filled, thetemperature of the mold, etc., and is preferably 2 to 20 sec, morepreferably 2 to 15 sec. If the holding time is too long, the skin layerover the surface of the resin becomes thick to substantially reduce thethickness of the expanded layer. This entails the likelihood that only afoamed body of low expansion ratio will be available.

According to the fourth feature of the invention, the period of timeduring which the cavity is held as filled with the resin immediatelyafter the completion of filling until the start of primary enlargementof the cavity is preferably 0.1 to 11 sec, more preferably 0.5 to 5 sec.The period of time during which the enlargement of the cavity isinterrupted between the primary and secondary cavity enlarging steps ispreferably 0.5 to 30 sec, more preferably 3 to 20 sec. If the durationof interruption is too short, the cell film will break as when theamount of cavity enlargement is great in the primary cavity enlargingstep, consequently entailing the likelihood that cells will join intolarger cells. If the duration of interruption is excessively long, theresin becomes cooled, with the ability of the resin to stretch exceedingthe expansion pressure, with the result that the resin will fail toexpand to the desired amount of cavity enlargement or will developsurface irregularities different from the shape of the mold.

The “rate of secondary cavity enlargement” according to the second,fourth or sixth feature of the invention is preferably 0.5 to 15 mm/sec,more preferably 5 to 10 mm/sec. If the rate is too low, the resinbecomes cooled during the enlargement of the cavity, with the ability ofthe resin to stretch exceeding the expansion pressure. As a result, theresin fails to expand to the desired increased cavity width or is likelyto develop surface irregularities different from the shape of the mold.When the rate of cavity enlargement is excessively great, the resinsurface separates from the mold once and is transferred onto the moldsurface again when the cavity is brought to a halt at the final positionof cavity enlargement. This is likely to give rise to the problem ofimpairing the appearance of the surface of the molding.

Next, the invention will be described below with respect to foamedbodies of thermoplastic resin.

The present invention provides as a twelfth feature thereof a foamedbody of a thermoplastic resin comprising an expanded inner layer portionand two surface layer portions having the inner layer portion sandwichedtherebetween and formed integrally therewith, the foamed body beingcharacterized in that:

-   each of the surface layer portions has a thickness of 0.1 mm to 3.0    mm and comprises a nonexpanded structure or an expanded structure up    to 10 μm, preferably to 5 μm in cell diameter, the expanded inner    layer portion having cells up to 6.0 mm in average cell diameter in    a direction parallel to the plane and including at least 70% of open    cells in a direction perpendicular to the plane,-   and the foamed body comprises an expanded structure up to 0.20 g/cm³    in average density.

The term a “direction perpendicular to the plane” refers to a directionperpendicular to the surface of the foamed body in the form of asandwich, i.e., the direction of thickness of the foamed body, and theterm a “direction parallel to the plane” refers to a direction parallelto the surface of the foamed body, i.e., a direction perpendicular tothe direction of the thickness.

Because of the cell form described, the expanded inner layer portion hascell walls providing a rib structure along the direction of thickness.Although the cell form is not limited specifically in section along thedirection parallel to the plane, a honeycomb form is desirable forimproved compressive strength. Such an expanded structure gives the bodyboth high rigidity against compression in the direction of thickness anda low density realized by the formation of open cells.

When the foamed body is bent, a tensile force acts on the front surfacelayer portion a1 and a compressive force on the rear surface layerportion a2 as shown in FIG. 9. In spite of the presence of the expandedinner layer portion b, the two surface layer portions a which have highrigidity enable the foamed body to exhibit high flexural rigidityagainst the above two forces.

Each of the surface layer portions has a thickness of 0.1 to 3.0 mm.From the viewpoint of ensuring both an improvement in flexural strengthand forming by expansion, the preferred thickness is 0.3 to 1.2 mm. Iftoo thin, the surface layer portion itself is low in rigidity,presenting difficulty in affording the contemplated flexural strength.An attempt to obtain a molding wherein the surface layer portions are atleast 3.0 mm in thickness encounters difficulty in increasing theexpansion ratio of the expanded inner layer portion, consequently givingan increased average density to the foamed body. If it is attempted toobtain a foamed body of high average density, the resin will not expandto the desired volume of enlarged cavity or the resin surface willdevelop irregularities different from the shape of the mold.

The expanded inner layer portion should be up to 6.0 mm in average celldiameter in a direction parallel to the plane to assure the foamed bodyof compressive strength in a direction perpendicular to the plane,whereas if an article acting to compress the foamed body has a smallarea of contact as when the foamed body of the invention is compressedby the finger or a ball-point pen, the average cell diameter ispreferably up to 4.0 mm.

If the cell diameter is not smaller than 6.0 mm or the compressingarticle has a small area of contact, the number of ribs contributing toan improvement in compressive strength will be smaller, failing toafford a satisfactory compressive strength.

The term “average cell diameter” refers to the average value of celldiameter in a direction parallel to the plane, as measured in a sectionparallel to the plane in the midportion of the thickness of the expandedinner layer portion of the foamed body. The measurement is done under amagnifying microscope. The range of measurement is 5 mm square when theaverage cell diameter is up to 100 μm, 15 mm square when the averagecell diameter is over 100 μm to up to 1 mm, or 50 mm square when theaverage cell diameter is over 1 mm. The average value of cell diametersat optional 30 locations within the range is calculated.

The ratio of open cells communicating with one another in a directionperpendicular to the plane (open cell ratio) should be at least 70%. Ifthis ratio is less than 70%, the body has a lower compressive strengthin a direction perpendicular to the plane.

An open cell ratio of at least 70% means that the value obtained bychecking cells for intercommunication by the air pycnometer method (ASTMD 2856) is at least 70%. For the determination of the value, theapparent density is measured, with the surface layer portions madeintegral with the expanded inner layer portion, and the actual volume ofthe sample was measured by the air pycnometer method, with the surfacelayer portions separated from the expanded inner layer portion.

The invention provides as a thirteenth feature thereof a foamed bodyaccording to the twelfth feature which is prepared from a polypropyleneresin serving as the thermoplastic resin. Thus the invention provides afoamed body of polypropylene resin comprising an expanded inner layerportion and two surface layer portions having the inner layer portionsandwiched therebetween and formed integrally therewith, the foamed bodybeing characterized in that:

-   the foamed body is at least 1.0 MPa in compressive strength in a    direction perpendicular to the plane, at least 0.025 GPa in modulus    in compression in a direction perpendicular to the plane and at    least 1.0 MPa in flexural strength, the foamed body having an    average density of up to 0.20 g/cm³.

The polypropylene resin is not limited specifically. Examples of usefulresins are a common polypropylene in the form of a homopolymer, randomcopolymer or block copolymer, metallocene polypropylene, a long-chainbranched polypropylene, and polypropylene having other componentincorporated therein by graft polymerization. These resins can be usedsingly, or at least two of them are used in combination.

When the average density is in excess of 0.2 g/cm³, it is difficult toobtain a lightweight foamed body.

The compressive strength is measured along the thickness of a test pieceat a compressing rate of the thickness×0.1 mm/min, the test piececomprising only an expanded inner layer portion with surface layerportions removed.

The invention provides as a fourteenth feature thereof a foamed body ofthermoplastic resin comprising an expanded inner layer portion and twosurface layer portions having the inner layer portion sandwichedtherebetween and formed integrally therewith, the foamed body beingcharacterized in that:

-   each of the surface layer portions has a thickness of 0.1 mm to 3.0    mm and comprises a nonexpanded structure or an expanded structure up    to 10 μm, preferably up to 5 μm, in cell diameter, the expanded    inner layer portion having cells up to 4.0 mm in average cell    diameter in a direction parallel to the plane and including at least    65% of cells which are 2 to 6 in flatness ratio, i.e., in the ratio    of the cell diameter thereof in a direction perpendicular to the    plane to the cell diameter thereof in a direction parallel to the    plane (former/latter), the foamed body comprising an expanded    structure up to 0.12 g/cm³ in average density.

Because of the flat cell structure described, the foamed body has thickcell walls in the direction of thickness and thin cell walls in thedirection of the plane. The body has high rigidity against compressionin the direction of thickness because of the thick cell walls in thisdirection and has great cell volumes since the cell walls are thin inthe direction of the plane. These features realize both high rigidityand a high expansion ratio at the same time. Furthermore, the closedcell structure affords a high heat-insulating property.

The foamed body has low heat-insulating properties when the expandedinner layer portion has cells which are greater than 4.0 mm in averagecell diameter in a direction parallel to the plane, when the flatnessratio is in excess of 6, or when the foamed body has an average densityof more than 0.12 g/cm³. In the case where the flatness ratio is smallerthan 2.0, or when the amount of cells with a flatness ratio of 2 to 6 isless than 65%, a satisfactory compressive strength is not available.

The invention provides as a fifteenth feature thereof a foamed body ofthermoplastic resin comprising an expanded inner layer portion and twosurface layer portions having the inner layer portion sandwichedtherebetween and formed integrally therewith, the foamed body being afoamed body of polypropylene characterized in that:

-   the foamed body is up to 0.070 W/mK in thermal conductivity, at    least 0.25 MPa in compressive strength in a direction perpendicular    to the plane, at least 0.004 GPa in modulus in compression in a    direction perpendicular to the plane and at least 1.0 MPa in    flexural strength, the foamed body having an average density of up    to 0.12 g/cm³.

If the average density is in excess of 0.12 g/cm³, it is difficult toobtain a lightweight foamed body.

The invention provides as a sixteenth feature thereof a foamed body ofpolypropylene resin according to any one of the twelfth to fifteenthfeatures which is characterized in that the foamed body is substantiallyfree from any of residues of a chemical foaming agent when the chemicalfoaming agent is used.

The term “residues” refers to both the unreacted components of thechemical foaming agent and the decomposition products thereof. Forexample in the case of azodicarbonamide (ADCA), the decompositionproducts of the chemical foaming agent are HDCA, urazol, cyanuric acid,isocyanuric acid, cyameride, ammonium cyanate, urea, etc. The expression“substantially free from any residue of a chemical foaming agent” asherein used means that when the components of the foamed body areanalyzed by IR, the residues are up to 50 ppm.

The thermoplastic resin foamed body according to the twelfth tosixteenth features of the invention can be produced, for example, by theprocess of the invention according to the second or fourth featurethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of injection molding apparatusfor use in a process for producing an foamed thermoplastic resin body ofthe invention.

FIG. 2 is a view in horizontal section showing a mold before the cavitythereof is enlarged.

FIG. 3 is a view in horizontal section showing the mold of FIG. 2 afterthe cavity thereof is enlarged.

FIG. 4 includes views in horizontal section showing a mold for producingfoamed thermoplastic resin bodies of the invention.

FIG. 5 is a view in horizontal section showing other cavity enlargingmeans.

FIG. 6 is a view in horizontal section showing a cavity as enlarged bythe cavity enlarging means shown in FIG. 4.

FIG. 7 is a graph showing the relationship between the rate of cavityenlargement and the time, as involved in the process for producing thefoamed thermoplastic resin body of the invention.

FIG. 8 is a side elevation showing the foamed body obtained.

FIG. 9 is a diagram for illustrating the forces occurring when an foamedthermoplastic resin body of the invention is bent.

FIG. 10 is a photograph showing in section an foamed thermoplastic resinbody obtained in Example 7.

FIG. 11 is a photograph showing in section an foamed thermoplastic resinbody obtained in Example 8.

FIG. 12 is a graph showing the relationship between the rate of cavityenlargement and the time, as involved in a conventional process forproducing an foamed thermoplastic resin body.

FIG. 13 includes views in horizontal section showing a conventional moldfor producing the foamed thermoplastic resin body.

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings.

FIG. 1 is a diagram showing an example of injection molding apparatus.Indicated at 1 in FIG. 1 is the injection molding apparatus, at 11 aresin plasticizing-kneading cylinder of the apparatus 1, at 12 apressure-resistant chamber provided above the cylinder 11 and positionedtoward the rear end thereof, at 13 a hopper disposed above the chamber12, and at 14 a cylinder for carbon dioxide for use as a foaming agent.The cylinder is connected to the pressure-resistant chamber 12 by achannel 142. A pressure regulating valve 141 is provided on the channel142. Indicated at 121, 122, 123 and 124 are valves. An injection moldingmold 2 comprises a stationary mold member 21 and a movable mold member22.

A polypropylene (HMS-PP, product of Montell-JPO, 127° C. in meltingpoint) serving as a thermoplastic resin was placed into a hopper 13 andfed to the chamber 12 with the valve 122 opened. The valves 122, 123,124 were thereafter closed, and carbon dioxide as adjusted to a pressureof 5.5 MPa by the pressure regulating valve 141 was introduced into thechamber 12 via the valve 121.

The interior of the pressure-resistant chamber 12 was maintained at acarbon dioxide pressure of 5.5 MPa and a temperature of 45° C. for 2hours to dissolve the carbon dioxide in the thermoplastic resin.

Incidentally, an inert gas can be dissolved in a thermoplastic resinunder a high pressure by dissolving the inert gas in the resin as meltedor,by dissolving the gas in the resin in a solid state. Either of thesemethods can be used, or the two methods are usable in combination.

The inert gas can be dissolved in the molten thermoplastic resin at ahigh pressure, for example, by mixing the gas with the resin within theinjection cylinder 11 via a bend provided at an intermediate portion ofthe cylinder. In this case, the molten thermoplastic resin serves as apressure seal.

The inert gas can be dissolved in the solid thermoplastic resin, forexample, by:

-   (1) dissolving the inert gas in the thermoplastic resin in the form    of pellets or a powder within a pressure container or the like in    advance, or-   (2) dissolving the inert gas in the thermoplastic resin by supplying    the gas to a region of the injection molding apparatus from the    hopper to a solid transport portion thereof.

In the case of the method (2) above, it is desirable to incorporate apressure-resistant seal structure into the screw drive shaft and thehopper so as to prevent the inert gas from evaporating off from theinjection cylinder 11.

The inert gas can be supplied from the gas cylinder 14 directly to theinjection cylinder 11, or the gas can be supplied as pressurized with anunillustrated plunger pump.

The thermoplastic resin containing carbon dioxide was fed from thepressure-resistant chamber 12 to the cylinder 11 set at 190° C. via thevalve 124. The resin as collected in a measuring portion toward thefront end of the cylinder 11 was injected into an initial cavity 3having a thickness of 3 mm via a runner 5 and a gate 4 as shown in FIG.2.

With reference to FIGS. 2 and 3, the movable mold member 22 comprises amovable piece 221 movable forward or rearward in the direction ofthickness of the cavity, and a wedge piece 222 movable upward ordownward by the operation of a hydraulic cylinder 223 connected to ahydraulic unit 224. The cavity 3 is diminished when the wedge piece 222is lowered as seen in FIG. 2 or is enlarged when the wedge piece 222 israised as shown in FIG. 3.

FIG. 4 shows an example of mold of the invention.

With reference to the mold 2 a shown in FIG. 4, an initial cavity 31 isformed between a stationary mold member 21 a and a movable mold member22 a, and one end of a movable piece 221 a of the movable mold member 22a is movable forward or rearward (51 or 52) within the cavity 31 alongthe direction of thickness of the cavity. As shown in FIG. 4(a), theinitial cavity 31 is in the form of a tube having a bottom and arecessed section along the direction of forward or rearward movement.When the initial cavity 31 is filled with molten resin as shown in FIG.4(b), a filled resin layer 40 is therefore formed which is tubular, hasa bottom and a recessed section in the direction of forward or rearwardmovement, and comprises a main body portion 41 and a tubular peripheralportion 42. When the movable piece 221 a is subsequently moved rearwardtoward the direction 52 as shown in FIG. 4(c), the central portion ofthe filled resin layer 40 expands following the retraction of themovable piece 221 a to form a foamed body 4, of desired shape.

The tubular peripheral portion 42 of the filled resin layer 40 can beheld at a high temperature by making the thickness t4 of the peripheralportion 42 larger than the thickness t3 of the main body portion 41.This makes the foamed body 4 easier to produce by enlarging the cavity31.

FIGS. 5 and 6 are diagrams for illustrating other example of means forexpanding a cavity 3. Instead of the enlarging means for the cavity 3shown in FIGS. 2 and 3 which means includes the wedge piece 222,hydraulic unit 224 and hydraulic cylinder 223, used as this enlargingmeans is an opening device for a mold 2 b of an injection moldingapparatus 1.

A movable piece 221 b is movable forward or rearward along with amovable mold member mount plate 15 by the operation of operating rod 16of a hydraulic cylinder 17 of the apparatus 1. FIG. 4 shows the cavity 3as diminished, with the movable piece 221 b moved forward along with themount plate 15. FIG. 5 shows the cavity 3 as enlarged, with the movablepiece 221 b moved rearward along with the mount plate 15.

FIG. 7 shows the relationship between the rate of cavity enlargement andthe time, involved in the process of the invention for producing foamedthermoplastic resin bodies. The cavity is enlarged at a rate of V1 in aprimary cavity enlarging step, i.e., during the period from the startuntil time T1, the enlargement is interrupted at time T1, and the cavityis enlarged at a rate V2 in a secondary cavity enlarging step, i.e.,from time T2 to time T3.

EXAMPLE 1 First and Third Features of the Invention

With reference to FIG. 4(a), a molten polypropylene resin was filledinto the initial cavity 31 in the form of a tube having a bottom and arecessed section along the direction of forward or rearward movement (3mm in width t3 along this direction of the main cavity width 34corresponding to the bottom portion thereof). The cavity 31 wasthereafter held in its shape as filled with the resin, the movable piece221 a was moved rearward by 12 mm by raising the wedge piece 222 withthe hydraulic cylinder 223 upon the average temperature of the resindropping to 180, 150 or 120° C. (2 sec, 9 sec or 22 sec, respectively,in holding time from the completion of filling until the cavityenlargement) to enlarge the cavity 31 to the increased final cavitywidth. The rate of enlargement of the cavity 31 at this time was 5mm/sec.

The average temperature of the molten resin when the movable piece 221 awas to be moved rearward was controlled by holding the initial cavity 31in its shape as filled with the resin for a specified period of time.The temperature of 190° C. for cavity enlargement listed in Table 1below is the temperature involved in the condition that the cavity washeld in shape as filled with the resin for 0 sec.

The foamed body of thermoplastic resin in the cavity 31 was thereaftercooled for 240 sec, and then the mold 2 was opened to withdraw thefoamed body 4.

Table 1 shows the result obtained by observing the foamed bodies thusprepared. TABLE 1 Temp. (° C.) when cavity is enlarged Cell sizeSmoothness 190 Coarse x 180 Coarse ∘ 150 Fine ∘ 120 Fine x∘: goodx: unacceptable[Evaluation]

Table 1 reveals that the foamed body of Example 1 had fine cells in thecase where the average temperature of the resin was 120° C. or 150° C.when the cavity was enlarged. When the average temperature of the resinwas in excess of the range [the melting point+30° C.] as in the case of180 or 190° C., coarse cells were obtained with large voids found in theinterior of the expanded resin body.

When the resin average temperature for enlarging the cavity was 150° C.or 180° C., the body exhibited high smoothness. However, when the resinaverage temperature for cavity enlargement was below the above range,e.g., 120° C., the thermoplastic resin failed to expand to the desiredshape and to provide a smooth-surfaced foamed body. The resin failed toexpand to the desired shape and to afford a smooth-surfaced foamed bodyalso when the resin average temperature was 190° C. for cavityenlargement.

EXAMPLE 2 Second and Fourth Features of the Invention

Primary Cavity Enlarging Step

With reference to FIG. 4(a), a molten polypropylene resin was filledinto the initial cavity 1 which was 3 mm in width t3 of the main cavitywidth 34. The cavity 31 was thereafter held in its shape as filled withthe resin for 0.5 sec, and the movable piece 221 a was moved rearward (4mm) to an intermediate position by raising the wedge piece 222 with thehydraulic cylinder 223 upon the average temperature of the resindropping to 185° C. The rate of enlargement of the cavity 31 was 5mm/sec in this primary cavity enlarging step.

Secondary Cavity Enlarging Step

The enlargement of the cavity was then interrupted at the intermediateposition, and the movable piece 221 a was thereafter moved rearward byraising the wedge piece 222 again with the hydraulic cylinder 223 uponthe temperature of the resin central portion in the direction ofthickness dropping to 190° C., 170° C., 150° C. or 120° C. (0 sec, 10sec, 14 sec or 31 sec, respectively, in cavity enlargement interruptingtime) to enlarge the cavity to an increased final cavity width of 26 mm.The rate of enlargement of the cavity 31 at this time was 10 mm/sec.

The foamed thermoplastic resin body in the cavity 31 was cooled for 360sec after this secondary cavity enlarging step, and the mold 2 wasopened to withdraw the foamed body 4.

Table 2 shows the result obtained by observing the foamed bodies thusprepared. TABLE 2 Resin central temp. (° C.) for secondary cavityenlargement Cell size Smoothness 190 Void x (no interrupt) 170 Fine ∘150 Fine ∘ 120 Fine x∘: goodx: unacceptable[Evaluation]

Table 2 reveals that the foamed body of Example 2 had fine cells whenthe resin average temperature for starting the primary cavity enlargingstep was 185° C. which was within the range of [the melting point to themelting point+60° C.] and when the temperature of the resin centralportion along the direction of thickness for starting the secondarycavity enlarging step was 170° C., 150° C. or 120° C. However, if theenlargement of the cavity was not interrupted between the primary andsecond cavity enlarging steps and when the temperature of the resincentral portion along the direction of thickness for starting thesecondary cavity enlarging step was 190° C., avoid occurred in thecentral portion of the foamed thermoplastic resin body obtained, withcoarse cells present in the other portion. Moreover, the thermoplasticresin foamed body failed to expand to the desired shape and to afford asmooth-surfaced foamed body.

In the case where the resin central portion temperature was 170° C. or150° C. when the secondary enlargement of the cavity was performed, thefoamed body had excellent surface smoothness. However, if the resincentral portion temperature was below the melting point, e.g., 120° C.,when the secondary cavity enlargement was done, the foamed thermoplasticresin body failed to expand to the desired shape and to afford asmooth-surfaced foamed body.

Example 3

The same procedure as in the primary cavity enlarging step of Example 2was performed except that in the primary cavity enlarging step, themovable piece 221 a was moved rearward upon the average temperature ofthe resin filled in the initial cavity 31 dropping to 195, 180 or 125°C.

The same procedure as in the secondary cavity enlarging step of Example2 was performed except that in the secondary cavity enlarging step, themovement of the cavity was interrupted until the temperature of thecentral portion of the resin in the direction of its thickness decreasedto 160° C.

Table 3 shows the result obtained by observing the foamed bodiesprepared. TABLE 3 Average temp. (° C.) for primary cavity enlargementCell size Smoothness 195 Coarse ∘ 180 Fine ∘ 125 Fine x∘: goodx: unacceptable[Evaluation]

Table 3 reveals that the foamed body of Example 3 had fine cells whenthe average resin temperature was 180° C. or 125° C. when the primarycavity enlargement was performed.

However, under the condition that the average resin temperature was inexcess of the range [the melting point+60° C.], e.g., 195° C., when thecavity was enlarged, the foamed body had coarse cells, with a hollowportion found in its central portion.

The foamed body was excellent in smoothness if the average resintemperature was 195° C. or 180° C. when the primary cavity enlargementwas done. However,if the average resin temperature was below the aboverange, e.g., 125° C., when the primary cavity enlargement was done, thefoamed thermoplastic resin body failed to expand to the desired shapeand to afford a smooth-surfaced foamed body.

EXAMPLE 4

The same procedure as in the primary cavity enlarging step of Example 2was performed except that in the primary cavity enlarging step, themovable piece 221 a was moved rearward to enlarge the cavity 31 at arate of 0.5, 5 or 10 mm/sec upon the average temperature of the resinfilled in the initial cavity 31 dropping to 180° C.

The same procedure as in the secondary cavity enlarging step of Example2 was performed except that in the secondary cavity enlarging step, themovement of the cavity was interrupted until the temperature of thecentral portion of the resin in the direction of its thickness droppedto 160° C.

Table 4 shows the result obtained by observing the foamed bodiesprepared. TABLE 4 Rate of primary cavity enlargement (mm/sec) Cell sizeAppearance 0.5 Coarse ∘ 5 Fine ∘ 10 Coarse x∘: goodx: unacceptable[Evaluation]

Table 4 reveals that the foamed body of Example 4 had fine cells whenthe rate of primary cavity enlargement was 5 mm/sec. Coarse cells wereobserved if the rate of primary cavity enlargement was outside the rangeof the invention.

The foamed body had an excellent appearance when the rate of primarycavity enlargement was 0.5 or 5 mm/sec.

EXAMPLE 5

The same procedure as in the primary cavity enlarging step of Example 2was performed except that in the primary cavity enlarging step, themovable piece 22 a was moved rearward upon the average temperature ofthe resin filled in the initial cavity 31 dropping to 180° C.

The same procedure as in the secondary cavity enlarging step of Example2 was performed except that in the secondary cavity enlarging step, themovement of the cavity was interrupted until the temperature of thecentral portion of the resin in the direction of its thickness droppedto 160° C., with the rate of enlargement of the cavity 31 set at 1, 10or 20 mm/sec.

Table 5 shows the result obtained by observing the foamed bodiesprepared. TABLE 5 Rate of secondary cavity enlargement (mm/sec) Cellsize Appearance Smoothness 1 Coarse ∘ x 10 Fine ∘ ∘ 20 Coarse x ∘∘: goodx: unacceptable[Evaluation]

Table 5 reveals that the foamed body of Example 5 had fine cells whenthe rate of secondary cavity enlargement was 10 mm/sec.

Coarse cells or hollow portions were observed if the rate of secondarycavity enlargement was 1 or 20 mm/sec.

The foamed body had an excellent appearance when the rate of secondarycavity enlargement was 1 or 10 mm/sec. Furthermore, an expanded moldingof excellent smoothness was obtained when the rate of secondary cavityenlargement was 10 or 20 mm/sec.

EXAMPLE 6

The same procedure as the primary cavity enlarging step of Example 2 wasperformed with the exception of filling in the primary cavity enlargingstep a molten thermoplastic resin into the initial cavity 31 shown inFIG. 4(a) which was 3 mm in the width t3 of the main cavity space 34 inthe direction of forward or rearward movement, 0, 3 or 12 mm in thewidth t4 of the peripheral cavity space 32, and 0, 5 or 15 mm in thelength t5 of the peripheral cavity space 32 in the direction of forwardor rearward movement, and moving the movable piece 221 a rearward uponthe average temperature of the resin filled in the cavity 31 dropping to180° C.

The same procedure as in the secondary cavity enlarging step of Example2 was performed except that in the secondary cavity enlarging step, themovement of the cavity was interrupted until the temperature of thecentral portion of the resin in the direction of thickness thereofdropped to 160° C.

FIG. 8 shows the foamed body obtained. Table 6 shows the thickness t7 ofa left side portion of the foamed body 4, the thickness t2 of the bodyfrom the gate portion to the lower end of the central portion, and thethickness t8 of a right side portion thereof. TABLE 6 Width WidthThickness Thickness Thickness t4 (mm) t5 (mm) t7 (mm) t2 (mm) t8 (mm) 00 11.2 24.0 11.0 3 5 23.7 26.0 24.0 12 15 26.0 26.1 25.9[Evaluation]

With an increase in the length t5 of the peripheral cavity space 32along the direction of forward or rearward movement, the differencedecreases between the thickness t2 of the central portion of the body 4and the thicknesses t7, t8 of opposite side portions thereof.

EXAMPLE 7 12th and 13th Features of the Invention

Primary Cavity Enlarging Step

With reference to FIG. 4(a), a molten polypropylene resin was filledinto the initial cavity, 31 which was 2 mm in the width t3 of the maincavity width 34. The cavity 31 was thereafter held in its shape asfilled with the resin for 5 sec, and the movable piece 221 a was movedrearward (2 mm) to an intermediate position by raising the wedge piece222 with the hydraulic cylinder 223. The rate of enlargement of thecavity 31 was 15 mm/sec in this primary cavity enlarging step.

Secondary Cavity Enlarging Step

The movement of the cavity was interrupted at the intermediate positionfor 0.5 sec, and the movable piece 221 a was thereafter moved rearwardby raising the wedge piece 222 again with the hydraulic cylinder 223 toenlarge the cavity to an increased final cavity width of 20 mm. The rateof enlargement of the cavity 31 at this time was 15 mm/sec.

The foamed thermoplastic resin body in the cavity 31 was cooled for 120sec after this secondary cavity enlarging step, and the mold 2 wasopened to withdraw the foamed body 4.

The characteristics of the foamed body thus prepared were measured.Table 7 shows the results.

As apparent from the photograph of a section shown in FIG. 10 (asmagnified at ×6 under a microscope), the foamed thermoplastic resin bodyobtained was composed of an expanded inner layer portion b and twosurface layer portions a having the portion b sandwiched therebetweenintegrally therewith.

COMPARATIVE EXAMPLE 1 Example Having Large Cells

A foamed body was prepared by the same procedure as in Example 7 withthe exception of holding the molten polypropylene resin containingcarbon dioxide in the cavity for 2 sec after filling and until the startof primary enlargement of the cavity and enlarging the cavity at a rateof 3 mm/sec in the primary cavity enlarging step.

Table 7 shows the results obtained by measuring the characteristics ofthe foamed body prepared.

COMPARATIVE EXAMPLE 2 Example with a Small Open Cell Ratio

A foamed body was prepared in the same manner as in Example 7 with theexception of holding the molten polypropylene resin containing carbondioxide in the cavity for 2 sec after filling and until the start ofprimary enlargement of the cavity, enlarging the cavity at a rate of 3mm/sec in the primary cavity enlarging step, interrupting theenlargement of the cavity for 3 sec between the primary and secondarycavity enlarging steps, and enlarging the cavity at a rate of 10 mm/secin the secondary cavity enlarging step.

Table 7 shows the results obtained by measuring the characteristics ofthe foamed body prepared. TABLE 7 Example 7 Comp. Ex. 1 Comp. Ex. 2Compressive strength (MPa) 1.70 0.25 0.82 Modulus in compression (GPa)0.050 0.016 0.020 Flexural strength (MPa) 7.0 6.1 1.2 Density (g/cm³)0.09 0.09 0.09 Average cell diameter (mm) 2.0 8.5 2.7 Open cell ratio(%) 87 75 42 Surface layer thickness (mm) 1.2 1.2 0.1 Expanded innerlayer thickness 17.6 17.6 18.8 (mm)

EXAMPLE 8 14th and 15th Features of the Invention

A foamed body was prepared in the same manner as in Example 7 with theexception of holding the molten polypropylene resin containing carbondioxide in the cavity for 11 sec after filling and until the start ofprimary enlargement of the cavity, enlarging the cavity at a rate of 10mm/sec in the primary cavity enlarging step, interrupting theenlargement of the cavity for 3 sec between the primary and secondarycavity enlarging steps, and moving the movable piece 221 a rearward toan increased final cavity width of 30 mm in the secondary cavityenlarging step to enlarge the cavity 31 at a rate of 5 mm/sec.

Table 8 shows the results obtained by measuring the characteristics ofthe foamed body prepared.

As apparent from the photograph of a section shown in FIG. 11 (asmagnified at ×4 under a microscope), the foamed thermoplastic resin bodyobtained was composed of an expanded inner layer portion b and twosurface layer portions a having the portion b sandwiched therebetweenintegrally therewith.

COMPARATIVE EXAMPLE 3 Example having Large Cells)

A foamed body was prepared in the same manner as in Example 7 with theexception of holding the molten polypropylene resin containing carbondioxide in the cavity for 7 sec after filling and until the start ofprimary enlargement of the cavity, enlarging the cavity at a rate of 10mm/sec in the primary cavity enlarging step, interrupting theenlargement of the cavity for 1 sec between the primary and secondarycavity enlarging steps, and moving the movable piece 221 a rearward toan increased final cavity width of 17 mm in the secondary cavityenlarging step to enlarge the cavity 31 at a rate of 5 mm/sec.

Table 8 shows the results obtained by measuring the characteristics ofthe foamed body prepared.

COMPARATIVE EXAMPLE 4 Example with Great Flatness Ratio

A foamed body was prepared in the same manner as in Example 7 with theexception of holding the molten polypropylene resin containing carbondioxide in the cavity for 10 sec after filling and until the start ofprimary enlargement of the cavity, enlarging the cavity at a rate of 10mm/sec in the primary cavity enlarging step, interrupting theenlargement of the cavity for 0.5 sec between the primary and secondarycavity enlarging steps, and moving the movable piece 221 a rearward toan increased final cavity width of 17 mm in the secondary cavityenlarging step to enlarge the cavity 31 at a rate of 5 mm/sec.

Table 8 shows the results obtained by measuring the characteristics ofthe foamed body prepared. TABLE 8 Example 8 Comp. Ex. 3 Comp. Ex. 4Compressive strength (MPa) 0.40 0.35 0.24 Modulus in compression (GPa)0.006 0.0036 0.005 Flexural strength (MPa) 2.1 1.0 2.1 Average density(g/cm³) 0.06 0.11 0.11 Thermal conductivity (W/mK) 0.041 0.072 0.086Average cell diameter (mm) 2.3 5.2 2.7 Cell diam. in thickness 3.0 2.46.9 direction/cell diam. in plane direction Surface layer thickness (mm)1.5 1.2 1.5 Expanded inner layer thickness 27.0 14.6 14.0 (mm)

The present invention has the following advantages.

Homogeneous foamed bodies having fine cells can be prepared by thefoamed thermoplastic resin body production process according to thefirst or third feature of the invention.

Homogeneous foamed bodies having a high expansion ratio and varying celldiameters can be prepared by the foamed thermoplastic resin bodyproduction process according to the second or fourth feature of theinvention.

Homogeneous foamed bodies having high dimensional accuracy, asmooth-surfaced excellent appearance and fine cells can be prepared bythe foamed thermoplastic resin body production process according to thefifth feature of the invention.

Homogeneous foamed bodies having high dimensional accuracy, asmooth-surfaced excellent appearance, a high expansion ratio and finecells can be prepared by the foamed thermoplastic resin body productionprocess according to the sixth feature of the invention.

Foamed bodies of stabilized quality and high expansion ratio can beprepared by the foamed thermoplastic resin body production processaccording to the seventh feature of the invention because the foamingagent used has high ability to effect expansion and can be dissolved inthe resin homogeneously.

Foamed bodies which are shaped as desired even at their peripheralportions can be prepared by the foamed thermoplastic resin body formingmold of the eighth feature.

The foamed thermoplastic resin body forming mold according to the ninth,tenth or eleventh feature of the invention is adapted to maintain theresin in its peripheral portion at a temperature higher than that of theresin on the movable piece and therefore to produce with good stabilityfoamed bodies which are shaped as desired even at their peripheralportions. Since the movable mold member can be moved at different timesfor enlargement by this process, it is possible to prepare homogeneousfoamed bodies of high expansion ratio and varying cell diameters.

The foamed thermoplastic resin bodies according to the twelfth andthirteenth features of the invention are extremely high in compressiverigidity and flexural rigidity, lightweight and excellent inheat-insulating properties.

The foamed thermoplastic resin bodies according to the fourteenth andfifteenth features of the invention are excellent in lightweightness andheat-insulating properties and have compressive rigidity and flexuralrigidity.

The foamed thermoplastic resin body according to the sixteenth featureof the invention is free from the residues of chemical foaming agent andusable for food uses. Especially when the thermoplastic resin ispolypropylene, the foamed body is suited to reuse through recycling andfavorably usable for food containers, heat-insulating buildingmaterials, house materials, motor vehicle components, etc.

INDUSTRIAL APPLICABILITY

The invention provides foamed bodies of thermoplastic resin having ahigh expansion ratio, desired shape and cells of desired diameter,especially fine cells. The foamed thermoplastic resin bodies provided bythe invention are exceedingly high in compressive rigidity and flexuralrigidity, lightweight and outstanding in heat-insulating properties.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled) 6.(canceled)
 7. (canceled)
 8. A mold for forming a foamed body of athermoplastic resin by filling the thermoplastic resin as melted andcontaining a foaming agent into a cavity defined by a stationary moldmember and a movable mold member of a mold and thereafter enlarging thecavity to expand the resin and produce the foamed body thereof, the moldbeing characterized in that: the movable mold member comprises a movablepiece (221 a) movable forward or rearward along the direction ofthickness of the cavity and an immovable piece outside the movablepiece, the movable piece being so shaped as to define a peripheralcavity space (32) by the immovable piece, an outer peripheral surface ofthe movable piece and a peripheral cavity wall (33) and a main cavityspace (34) by a front end face of the movable piece and a cavity wall(35) opposed thereto when the movable piece is moved forward.
 9. A moldfor forming a foamed body of a thermoplastic resin according to claim 8which is characterized in that the length (t5) of the peripheral cavityspace (32) in the direction of forward or rearward movement is greaterthan the width (t3) of the main cavity space (34) in the direction offorward or rearward movement.
 10. A mold for forming a foamed body of athermoplastic resin according to claim 8 which is characterized in thatthe length (t5) of the peripheral cavity space (32) in the direction offorward or rearward movement is 50 to 70% of an increased final cavitywidth (t6).
 11. A mold for forming a foamed body of a thermoplasticresin according to claim 8, which is characterized in that the width(t4) of the peripheral cavity space (32) is two to four times the width(t3) of the main cavity space (34).
 12. (canceled)
 13. (canceled) 14.(canceled)
 15. (canceled)
 16. (canceled)
 17. A mold for forming a foamedbody of a thermoplastic resin according to claim 9 which ischaracterized in that the length (t5) of the peripheral cavity space(32) in the direction of forward or rearward movement is 50 to 70% of anincreased final cavity width (t6).
 18. A mold for forming a foamed bodyof a thermoplastic resin according to claim 9 which is characterized inthat the width (t4) of the peripheral cavity space (32) is two to fourtimes the width (t3) of the main cavity space (34).